sidus

Sidus, sideris is third declension neuter, an s-stem, so the s of the nominative turns to r between vowels: sidus, sideris, sidere, sidera, siderum. Same pattern as corpus and genus.

Strictly, stella is the single star and sidus the constellation or the group, though the poets ignore the distinction whenever the metre wants them to. The Romans took the stars seriously as agents, not decoration, and two ordinary Latin verbs carry that with them: considero and desidero both have sidus inside, one meaning to study the stars closely, the other to stop seeing them, hence to miss something. English got sidereal from it, and disaster by a longer road through Italian, an ill star.